1. Field of the Invention
The present invention is directed to a sensor for measuring the torque acting on a rotating shaft, and in particular to such a sensor including a capacitive transducer which measures a change in the distance or spacing between points on the shaft and converts the measurement into an electrical signal.
2. Description of the Prior Art
Precise, non-contacting measurement of the output torque, and thus of the mechanical power, of a rotating shaft is currently one of the most urgent sensor problems in energy and automation technology. A signal corresponding to the output torque is useful for monitoring and controlling drive systems, as well as for optimizing the efficiency of energy conversion systems.
Currently known methods for non-contacting measurement of torque make use either of the acquisition of a mechanical tension generated due to the moment occurring at the shaft surface, or measurement of the torsion.
It is known to apply wire strain gauges to the shaft for converting the mechanical tension into an electrical signal, with the measured signal being taken from the rotating shaft at a carrier frequency. This method has been predominately employed in the laboratory, because attachment of wire strain gauges to the shaft is relatively complicated, and the measuring transmission is very costly.
It is also known to mechanically couple an amorphous metal layer to the shaft to be measured, and to measure the change in the permeability of the amorphous metal layer which occurs due to mechanical tension, which is proportional to the torque acting on the shaft. This known method, however, requires some means for mechanically attaching the amorphous metal layers to the shaft.
The torsion of a shaft, for the usual range of shaft diameters and torques, is extremely slight. An electric motor shaft having a diameter of 70 millimeters, for example, experiences torsion at nominal torque values of only a few micrometers at a measuring distance of 30 millimeters. For intensifying the torsional path, a longer measuring shaft is usually connected to the end of the shaft to be measured, and the torsion is taken from the measuring shaft via inductive systems. This method, however, can usually not be applied "in the field" because there is usually no space available for the additional measuring shaft. A proposal has been made in the periodical VDI-Nachricten, No. 20, May 15, 1987 to make the torque measurable by measuring the torsion at the end of the shaft to be measured, by converting the torsional path on the shaft into an axial motion with a mechanical lever system. The axial displacement, which will be in the range of tenths of a millimeter, is then acquired in non-contacting fashion by an inductive system. The axial measuring interval on the shaft corresponds approximately to twice the shaft diameter. This method, however, requires a relatively complex mechanism, and the obtainable precision is about 5%, which is not sufficient for many applications.
A sensor for non-contacting measurement of the torque experienced by a rotating shaft is described in British specification 2 195 183, wherein measurement of the torsional path is made using a capacitive transducer. A capacitor arrangement having electrode structures which are arranged at a radial spacing relative to each other, and proceeding parallel to the shaft surface, is provided as a variable capacitance. A change in capacitance, corresponding to the torsion (the transmitted torque) is obtained by changes in the electrode surface. The electrode structures are comb-like and are arranged on the shaft surface and on a tube surrounding the shaft at a radial distance therefrom, with the tube being connected to the shaft at an axial distance from the capacitor arrangement. This axial distance corresponds to approximately 5 times the value of the shaft diameter, since the slight changes in capacitance which are involved require that the torsional path being measured be made relatively long.